About 78,000 people live where a Mount Rainier mudflow would run

The U.S. Geological Survey counted 78,049 residents, or about 78,000 people, inside a modeled Mount Rainier mudflow zone that crosses four Washington counties. The same zone holds 59,678 employees and 3,890 businesses. Authors Nathan Wood and Christopher Soulard published the tally in Scientific Investigations Report 2009-5211, and it measures who and what sits in the path, not who would be hurt.

Save more, waste less: The Cool Down’s newsletter sends smart, simple changes that help you and the planet in your everyday life.

Subscribe to The Cool Down →

We may earn a commission if you sign up.

Which zone matters here. The report draws its boundary from the Electron Mudflow, a lahar that ran down the Puyallup River about 500 years ago after a slope on Rainier’s west flank failed. The zone is a modeled inundation area for that one past event, so it is a single scenario and not a map of every flow the volcano could produce. Other USGS counts use other hazard maps and reach other totals, and the 78,049 belongs to this report’s zone only.

The Electron Mudflow zone behind the 78,049 count

The USGS report was prepared with the Washington State Military Department’s Emergency Management Division and first posted on October 6, 2009. Its purpose was to show how exposed communities in King, Lewis, Pierce and Thurston Counties are to lahars, the fast-moving volcanic debris flows, so local officials could plan risk reduction. Of the 78,049 residents, about 11 percent are over 65 and about 39 percent of households are renter-occupied. Roughly 21 percent of residents live outside cities and unincorporated towns. The report states no data year for the population count, so it is best read as a snapshot from the time of the study.

The choice of the Electron Mudflow as the template is not arbitrary. A later USGS modeling study, Open-File Report 2021-1118 by David George, Richard Iverson and Charles Cannon, sizes its worst-case flows after the roughly 260 million cubic meter Electron Mudflow of about 500 years ago. That study simulated eight hypothetical lahars from the west side of the mountain into the Puyallup and Nisqually River valleys, and found that a worst-case flow from the Sunset Amphitheater could reach the Orting lowlands about an hour after the slope failed.

Puyallup, Sumner, Fife and Orting: where exposure concentrates

Of the 27 communities the report studied, the city of Puyallup has the most people and assets in the zone. The highest percentages of people and assets belong to Carbonado, Fife, Orting and Sumner, which are smaller places where the zone covers a larger share of the community. Combining number and percentage into a composite index, the report ranks Puyallup, Sumner and Fife highest.

The authors warn that exposure varies widely, and that a raw count can mislead in both directions. Some communities could suffer large losses relative to only a small part of their population or assets, while others could be devastated by losses that look small in absolute terms. That is why the report scores communities three ways, by count, by percentage and by a composite of the two, instead of ranking them on one number.

Workers, businesses and property in the path

The zone is also a workplace district. The report counts 59,678 employees, about 4 percent of the four-county labor force, working mainly in manufacturing, retail, transportation and warehousing, wholesale trade and construction. The 3,890 businesses generate about $16 billion in annual sales, roughly 7 percent of the four-county total. Tax parcels in the zone are worth a combined $8.8 billion, about 2 percent of the study area, according to the report.

The report also lists the sites that make a mudflow zone more than a residential count: schools, child day-care centers, hotels, religious organizations, police and fire stations, Mount Rainier National Park, the Puyallup Fairgrounds, and major road and rail corridors, including those serving the Port of Tacoma. The authors note that occupancy is not known for each critical or public site, so the site-level exposure is incomplete.

Other Rainier counts and what warning time looks like

A separate figure circulates for the same volcano. In a 2022 feature on lahar modeling, the USGS says a 2015 study found over 90,000 people live in Mount Rainier lahar hazard zones, along with over 50,000 employees at about 3,800 businesses. That is a different document covering hazard zones in the plural, so the 78,049 is neither a subset nor an update of it. The same feature states that the “greatest danger may not be a volcanic eruption, but instead large mudflows called lahars.”

Warning time is the other half of the exposure story. The USGS account of a March 21, 2024 evacuation drill says more than 45,000 students took part, and about 13,000 of them walked up to 2 miles to reach spots outside the mapped lahar zone. Its D-Claw modeling indicates a large lahar could reach Orting in as little as 50 minutes. The broader catalog of Rainier hazards, of which lahars are one, is the Volcano hazards from Mount Rainier, Washington, revised 1998. Those warning figures come from later USGS work and answer a different question from the 78,049 resident count, which is a tally of exposure and not a timetable.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

Save more, waste less: Subscribe to The Cool Down →

We may earn a commission if you sign up.